A sample of an ionic compound NaA, where A- is the anion of a weak acid, was dis

Question

A sample of an ionic compound NaA, where A- is the anion of a weak acid, was dissolved in enough water to make 100.0 mL of solution and was then titrated with 0.139 M HCl. After 500.0 mL of HCl was added, the pH was measured and found to be 4.88. The experimenter found that 1.00 L of 0.139 M HCl was required to reach the stoichiometric point of the titration. (a) What is the Kb value for A-? (b) Calculate the pH of the solution at the stoichiometric point of the titration. Please show your work and if you can, put it in the right significant figures.

Explanation / Answer

moles HCl = 1.00 L x 0.103 M = 0.103
= moles A-

moles HCl = 0.500 L x 0.103 = 0.0515
A- + H+ >> HA
moles A- = 0.103 - 0.0515 = 0.0515
moles HA = 0.0515
because the moles are the same also the concentrations will be the same

pKb = 9.36

pOH = pKb + log 1

pOH = 9.36
pH = 4.64

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